This invention relates to telephone systems in which a looped pulse-code modulated (PCM) trunk line extends outwardly from a central office, and in which one or more subscribers are connected to the trunk line at repeater locations.
In the prior art, it has been conventional to extend a PCM trunk line from a central office to a remote concentrator to which individual subscribers at the remote location are connected by two-wire pairs. With this system, a PCM trunk line (usually thought of as a T1 line) was able to serve only one remote location. In applications where subscribers are scattered along a T1 line, it has long been felt desirable to provide some kind of system in which the subscribers can access the T1 line directly at any repeater location.
Attempts to provide such apparatus have been made by various designers. For example, such techniques have been described in the following documents:
(1) "A Proposal for the Introduction of Digital Techniques into Local Distribution" by Ithell & Jones, Proceedings of the 1978 International Zurich Seminar on Digital Communications, IEEE Cat. No. 78 CH 1325-0 ASST, pp. 75.1-6;
(2) "An Integrated Digital Subscriber Multiplexing and Switching System for Mixed Circuit and Packet Communication" by Pederson, Conference Record of the 1978 International Symposium on Subscriber Loops and Services, IEEE Cat. No. 78 CH 1279-9 COM, pp. 156-160;
(3) "An Approach to Multiservice Subscriber Loop System Using Packetized Voice/Data Terminals" by Tsuda, Yatsuboshi, Yamauchi and Hattori; Conference Record of the 1978 International Symposium on Subscriber Loops and Services, IEEE Cat. No. 78 CH 1279-9 COM, pp. 161-165;
None of the foregoing publications describe, however, a simple, centrally controlled system in which any subscriber connected to any repeater along the line can access any free channel on the line or be accessed by any channel on the line, and which is capable of operating in an all-digital mode suitable for use with digital telephones and high-speed data sets at the subscriber locations.